I can either aim at the sky directly above me and at the ground directly below me. When I move mouse cursor a little bit down player starts looking at floor, when I move camera a little bit up player starts looking at the sky.
I have tried slowing down mouse movement at the minetest.config file, but that just makes camera movement slower (transition between looking at sky or at the ground is slower). Also I've tried changing video_driver setting to opengl.
I'm using Fedora 24 with Gnome Wayland.